This invention relates to a drain valve and, more particularly, to a drain valve which is especially useful for draining liquid from a closed vessel whose interior is under a vacuum at the time the valve is first opened. One such vessel forms part of a fuel/water separator or filter for an internal combustion engine. As fuel flows through the filter element, water is separated therefrom and collects at the bottom of the vessel. Periodically, water and other contaminants must be drained from the vessel. Rapid and complete draining can be effected only if ambient air is admitted into the vessel to break the vacuum therein.
Drain valves which are particularly adapted for use in fuel/water separators are disclosed in Wilson U. S. Pat. No. 4,314,689; Matheson U. S. Pat. No. 4,440,193 and Cudaback U. S. Pat. No. 4,708,171.These patents and others are summarized in Herman et al U. S. Pat. No. 4,893,651.
The Herman et al patent discloses a valve in which a stem is threadably received in a nut which is welded to the lower end of the vessel, the stem being formed with air and liquid passages which are isolated from one another. When the valve is closed, a gasket on the lower end portion of the stem seats against the weld nut to seal the passages and the vessel. The valve is opened by rotating the stem to move the gasket downwardly away from the weld nut and to enable the two passages to communicate with atmosphere. Air flows into the vessel via one of the passages to break the vacuum while liquid in the vessel drains from the other passage.
While the Herman et al valve is of relatively simple construction and is described in the patent as having only one sealing gasket, the valve in fact has two potential leak paths. The first is at the gasket while the second is at the joint between the weld nut and the vessel. Since there is no gasket between the nut and the vessel, leakage will occur if an inferior weld is made or if the weld fails.